1. Field of the Invention
This invention relates to printing plates, which are imaged by imagewise application of toner to a receiving layer, to methods of preparing such printing plates, and to methods of printing using such printing plates. More particularly, the printing plates of this invention are prepared by applying a composition comprising a water-soluble or water-dispersible polymer to a substrate surface to provide the surface with a polymer coating. The coated substrate is electrostatically imaged using a toner composition which is applied to the polymer coating, and the imaged substrate is heated to fuse the toner composition to the substrate polymer coating. The toner functions as an oleophilic, ink accepting image area and the non-imaged areas function as hydrophilic, water-accepting ink repellant background areas. The imaged plate is thereafter contacted with an appropriate solution to remove the non-imaged portion of the polymer coating which is unprotected by the fused toner composition, without the need for a chemical development step.
2. Background Information
The manufacture of printing plates, including printing plates used in lithographic printing processes, using electrostatic imaging techniques is well known in the art. In such methods, the fixed toner images are the olephilic ink receptive portions of the plate, and upon contact of the plate with an appropriate ink or ink-containing solution, the desired ink image may be transferred, or xe2x80x9coffset,xe2x80x9d from the plate to an appropriate medium, such as a rubber blanket, which is then used to print onto a medium such as paper. Examples of methods of preparing printing plates which are electrostatically imaged include:
U.S. Pat. No. 3,315,600, which discloses a method for preparing a printing plate in which a support having a hydrophilic surface is provided with a covering layer, the covering layer is electrostatically imaged using a toner composition, the image is fused or fixed via heating, and the covering layer is removed from the non-imaged areas by means of an aqueous solution. There is no disclosure regarding the thickness of the coverage layer.
U.S. Pat. No. 4,444,858, which discloses a method of preparing a lithographic printing plate in which a metal substrate is coated with a synthetic resin layer, and a toner image formed on a photosensitive sheet by an electrophotographic process is transferred and fixed to the synthetic resin layer. An organic solvent is used to remove the non-imaged areas of the resin layer, which are not covered by the fixed toner image.
U.S. Pat. No. 4,457,992, which discloses an etchable electrophotographic printing plate comprising an electroconductive support coated with a light-sensitive photoconductive zinc oxide and a sensitizing dye dispersed in an organic resin binder. Such plates are typically referred to as xe2x80x9corganic photoconductorxe2x80x9d or xe2x80x9cOPCxe2x80x9d plates. The coating is applied to the substrate as a thin film (i.e. about 5-50 xcexcm), and dried to remove substantially all of the solvent. The resulting plate may be imaged with electrostatic toner, and the non-imaged portions of the coating are removed via a basic aqueous solution. The plate may thereafter optionally be heated to enhance plate endurance. The toner-receiving, polymeric coating of the present invention is not photoconductive.
U.S. Pat. No. 6,025,100, which discloses a printing plate prepared by transferring a toner image to an image receiving element which is a support having an image receiving layer thereon. The layer contains a hydrophilic binder, TiO2 particles, and a matting agent, and the layer is cross-linked with hydrolyzed tetramethyl silicate or hydrolyzed tetraethylsilicate, thereby serving as the non-image area.
U.S. Pat. No. 6,014,929, which discloses a lithographic plate having a rough substrate and a releasable interlayer applied to the rough substrate surface. A radiation-sensitive layer may be applied to the interlayer. Alternatively, image-forming material may be imagewise transferred to the interlayer by an electrostatic process. The average coverage of the interlayer lies in the range of 0.001 to 0.2 g/m2, preferably 0.004 to 0.04 g/m2.
U.S. patent application Ser. No. 09/706,521 is directed to a printing plate prepared by applying an alkali soluble composition comprising at least one polymer to a hydrophilic substrate surface, imaging the coated substrate electrostatically with a toner composition, heating the imaged substrate to a temperature greater than the glass transition temperature of the toner, removing the portions of alkali soluble polymer which are not protected by the toner, and heating the imaged substrate to a temperature greater than the glass transition temperature of the toner a second time.
Electrostatically imaged printing plates are known to have several disadvantages. For example, if the underlying substrate is initially treated to be hydrophilic, it is often difficult to preserve the initial hydrophilic treatment of the substrate due to subsequent handling of the substrate and various factors which contribute to degradation of the hydrophilic surface such as fingerprints, scratches, presence of contaminants, and oxidation.
Another problem associated with electrostatically imaged printing plates is that the toner composition used typically yields inherently poor quality of the electrostatic image. The toner composition is composed of individual toner particles which are melted upon the intended substrate. If the substrate is untreated, the toner-imaged plate often results in a non-solid image with poor image properties and other practical disadvantages. This is believed to be due to the characteristics of the melting toner particles during the fusion process in terms of melt, flow, coalescence and adhesion, as well as the coalescence of the individual toner particles to one another during the imaging and fusion processes.
In view of the foregoing, it is one object of this invention to provide an electrostatically imaged printing plate which is capable of high quality and multi-color printing wherein image formation is enhanced due to improved toner melt, toner flow, coalescence and adhesion of the melting toner onto the substrate as well as enhanced coalescence of individual toner particles to each other to form a more solid image. It is another object of this invention to provide a method of preparing such a printing plate. It is yet another object of this invention to provide a method of printing using such a printing plate. It is another object of this invention to provide a method of preparing a printing plate that does not require a chemical development step. Accordingly, the printing plate of this invention is developable xe2x80x9con-pressxe2x80x9d, i.e., development takes place during the printing stage with an aqueous or fountain solution. It is a feature of this invention that the polymer coating employed in this invention advantageously controls the flow of melting toner and enhances image formation. The printing plate of this invention also advantageously may be employed for high quality printing applications such as four-color newspaper printing, book printing, financial and other small format printing, and the like. Other objects, features and advantages of this invention will be apparent to these skilled in the art.
A printing plate is prepared by a process comprising:
(a) applying a composition comprising at least one polymer to a substrate surface to provide the surface with a water-soluble polymer coating;
(b) imaging the coated substrate with a toner composition comprising toner particles by imagewise applying the toner composition to the polymer coating, wherein the polymer coating has a softening temperature TS;
(c) heating the imaged substrate to a temperature TF which is equal to or greater than TS; and
(d) contacting the imaged substrate with a solution capable of removing the non-imaged portion of the polymer coating,
wherein the process does not comprise a chemical development step.
In a preferred embodiment, the substrate is an aluminum substrate having a hydrophilic surface which receives the polymer coating. The polymer composition may be a polyethylene glycol having a molecular weight in the range of 1000-10,000, preferably 2500-6500. The total polymer coating weight is in the range of 0.25-2.0 g/m2, preferably 0.30-1.3 g/m2, more preferably 0.35-0.9 g/m2, most preferably 0.4-0.6 g/m2. The imagewise applied and fused toner is insoluble in the solution used to remove the non-imaged portion of the polymer coating in comparison to the polymer coating itself. The printing plate of this invention is capable of long runs on press with good image quality.